Two new ternary lanthanide antimony chalcogenides: Yb4Sb2S11.25 and Tm4Sb2Se11.68 containing chalcogenide Q2- and dichalcogenide (Q2)2- anions

Dark red and dark brown crystals of Yb4Sb2S11.25 and Tm8Sb4Se11.68, respectively, were obtained from the reaction of the elements in Sb(2)Q(3) (Q=S, Se) fluxes. Both non-stoichiometric compounds are orthorhombic and crystallize in the same space group Pnnm, with two formula units per unit cell (a=12.446(2), b=5.341(1), c=12.058(2) for sulfide and a=13.126(2), b=5.623(1), c=12.499(2) for the selenide). Their crystal structures are dominated by lanthanide-chalcogenide polyhedra (CN = 7 and 8), which share corners, edges, triangular- and square-faces to form a three-dimensional framework embedding antinomy cations. The latter are coordinated by three sulfide anions with 5(1+2+2) secondary contacts forming basically infinite chains running along [0 1 0]. The chalcogens in both compounds form chalcogenide Q(2-) and dichalcogenide (Q(2))(2-) anionic units. The optical analysis made on those compounds shows that both are semiconductors with band gap of 1.71 and 1.22 eV for Yb4Sb2S11.25 and Tm4Sb2Se11.75. respectively. (C) 2012 Elsevier Inc. All rights reserved.